The present invention pertains generally to slew rate control of signal drivers in integrated circuits, and more particularly to a novel tristateable output driver with controllable slew rate for integrated circuit input/output pads.
Integrated circuits are commonly packaged as chips. An integrated circuit communicates with devices external to the chip via input and output signal pads. Inside the chip, the signal pads are connected to signal receiver and signal driver circuitry, as appropriate, to receive incoming signals or to drive outgoing signals.
The signal pads on a chip are connected to the packaging of the chip (e.g., a pin, wire bond, solder ball, etc.) which is then typically connected to respective signal traces on a printed circuit board. The signal traces may connect the chip to other integrated circuit chips, electronic devices, or connectors on the printed circuit board that connect to external (i.e., off-board) devices.
In some integrated circuit applications, it is desirable to reduce the slew rate (i.e., the rise and fall times) of output signals. For example, some applications require reduced electro-magnetic interference (EMI). It is well known in the art that EMI emitted by a signal is related to its change in voltage with respect to time, whereby faster signal rise and fall times result in increased EMI. In applications that require low EMI, one obvious solution is to shield the output signals; however, shielded components (e.g., shielded boards, cables, connectors) add to the cost of the hardware. Another solution, as provided by the output driver of the present invention, is to reduce the slew rate on the edges of the signal transitions in order to directly reduce the actual EMI generated by the signal transition, and therefore eliminate or reduce the need for shielding.
In other or the same applications, it is desirable to reduce the noise on the power supplies. Fast signal transitions result in current spikes on the power supplies, resulting in greater supply noise. Conversely, by reducing the slew rate of the signal, causing it to transition over a longer of period of time, the magnitude of the current spikes, and therefore the noise, on the power supplies is reduced.
In other or the same applications, where the electrical length of the signal traces or busses are long compared to the signal rise/fall times, the traces/busses begin to assume transmission line characteristics (including parasitic resistance, capacitance, and inductance), and require impedance matching in order to avoid signal reflections. However, if the signal rise/fall time is long compared to the electrical length of the transmission line, such that the signal reflections are received at the source while the signal is still rising/falling, then the effects of the signal reflections are only minimal and essentially negated (since the voltage value of the reflections received at the source are relatively close in value to the voltage being driven by the source when the reflections reach the source). In order to lengthen the signal rise/fall time relative to the length of the transmission line, slew rate control of the signal driven by the output driver is necessary.